In substrate processing chambers such as those shown in the U.S. Patent to Zhao et al. (U.S. Pat. No. 5,558,717--CVD Processing Chamber), the movement of the substrate into and out of the chamber for processing involves several motions. One of the motions is to raise and lower the pedestal/heater with attached stem. This is the last motion before processing the wafer and the first motion after processing the wafer in the processing chamber. An example of such a lift mechanism is pictured in the U.S. Pat. No. 5,558,717 (which is expressly incorporated by reference herein). Larger and larger substrates and processing chambers are being developed used to increase the number of surface elements which are simultaneously processed on the surface of a substrate, to raise the production throughput. The use of such larger substrates and chamber elements require greater precision in the mechanical alignment and motion to maintain process uniformity.
An example of a type of mechanism 42 used in the prior art is shown in FIG. 1. A processing chamber 30 contains a substrate 32 to be processed. The substrate 32 rests on top of a pedestal/heater 36. During the transfer of the substrate 32 to and from its processing location in the processing chamber 30, lift pins 34 extend from the heater/pedestal 36 to raise the substrate 32 up from the top surface of the pedestal 36. The lift pins 34 allow a robot blade (not shown), for supporting the substrate during its transfer into and out of the chamber, to pass under the substrate supported on the extended lift pins 34 so that in a coordinated motion of raising and lowering the lift pins and insertion of the robot blade the substrate is transferred from and to a precise location on the surface of the pedestal. Once the pedestal is raised to its processing position, processing of the substrate supported on the top surface of the pedestal takes place.
Movement of the pedestal 36 is controlled by the motion of its stem 38. The stem is clamped and sealed to a stem carrier bracket 50 supporting the stem 38 and pedestal. The stem carrier bracket 50 is supported by a lift mechanism support structure/column 58 through engagement with a linear bearing track 60 and a threaded opening or nut engaged with a vertical drive screw 64 whose rotation raises or lowers the pedestal assembly. The generally vertical lifting mechanism support structure/column 58 is integral with a lift system connection plate 44 which is connected by a series of adjustment studs 46 to the bottom wall 40 of the processing chamber 30.
The angular orientation of the pedestal 36 and as a result the orientation and precise positioning of the substrate 32 in the processing chamber 30, is set and adjusted by the movement of the nuts or other adjustment members on the adjustment studs 46. A triangular arrangement of connection studs (for example see FIG. 8 of U.S. Pat. No. 5,558,717) provides one configuration for adjustment.
The stem opening in the bottom wall 40 of the processing chamber 30 is closed and sealed by a bellows assembly fixed to the bottom wall 40 of the processing chamber. The bellows assembly includes a fixed seal plate 47 connected to an upper part of an upper bellows 48a. An intermediate stem guide bracket 52 is sealed between the upper bellows 48a and the lower bellows 48b. The bottom of the lower bellows 48b is sealed to the top of the stem carrier bracket 50, which seals the end of the stem 38.
The space outside the stem 38 above the seal to the stem carrier bracket 50 is under vacuum and is sealed by the bellows assembly. The motion of the bellows 48a, 48b is guided by the intermediate guide bracket 52 which is also engaged with the linear bearing track 60. The vertical forces generated by the compression of the bellows 48a and 48b as the stem carrier bracket 50 moves causes the intermediate guide bracket 52 to move to balance the force on it.
In the arrangement shown, the stem 38 is moved in a vertical direction by moving the stem carrier bracket 50 guided by a bearing truck 62 engaging the bearing track 60 on the lift mechanism support structure/column 58. A threaded opening in the stem carrier bracket 50 engages the drive screw 64. In this configuration the drive screw 64, held by a duplex bearing 68, is rotated by a drive motor 76 through a drive pulley 72, drive belt 74, and driven pulley 70. The stem carrier bracket 50 moves the stem 38 up and or down depending on the direction of screw rotation. If the lift assembly components described were perfectly aligned to one another and made of perfectly rigid materials, such an assembly would operate consistently and easily to repeatably locate the pedestal and substrate at a particular location in the processing chamber. However, alignments are imperfect and deflections vary depending on the forces on the stem carrier bracket 50. At ambient conditions the weight of the pedestal and stem, and the force compressing the bellows tends to bend the end of the stem carrier bracket 50 down. In contrast when the processing chamber is evacuated, the atmospheric pressure outside the processing chamber tends to push the stem into the chamber rather tending to bend the stem up. Similarly the force of the drive screw 64 engaging the threaded receiving hole (or nut) on the stem carrier bracket 50 can be distorted as shown by FIGS. 3A, 3B, and 3C. In addition, binding between members because of misalignment and bending due to distortion can also occur. For the stem carrier bracket 50 to move uniformly and precisely throughout its range of travel along the bearing track 60, the forces on the stem carrier bracket 50 along the track 60 must be uniformly and consistently applied.
To avoid binding between the bearing track and the drive screw the distance 80 (shown in FIG. 3A), between the sliding axis of the bearing track 60 and the center line 66 of the drive screw 64 must be maintained.
In this configuration the duplex bearing 68 and its positioning controls the alignment of the drive screw 64. Therefore any variation in the mounting of the duplex bearing 68 within the lift mechanism support structure/column 58 will result in the misalignment between the bearing rail 60 and the centerline of the drive screw 64. Because the location of the screw receiving opening relative to the bearing truck 62 is fixed, any misalignment between the bearing track 60 and centerline 66 of the drive screw 64 will tend to cause binding between the two through the stem carriers bracket 50 as it moves along the track 60 and the drive screw 64.
In addition to binding because of misalignment, an angular misalignment between the two pieces also results. To accommodate the angular misalignment either the stem carrier bracket must bend to accommodate tightly engaged threads in the threaded rod receiving opening in the stem carrier bracket 50 or the threads (or nut) mounted therein must be loosely cut or mounted to accommodate expected angular variations due to misalignment. Sloppiness in the thread clearance can result in a free play distance as motion and forces exerted on the stem carrier bracket change directions, for example when the force between the end of the stem carrier bracket 50 supporting the stem 38 goes from a downwardly bending force as shown in FIG. 3B (which results from the normal weight of the pedestal and stem when the processing chamber is at atmospheric pressure) to an upwardly directed force 104 as shown in FIG. 3 bending the stem carrier bracket 50 upwards (when the evacuated processing chamber causes the external atmospheric force on the bellows and end of the stem to push the stein and pedestal into the processing chamber). In each instance forces tending to move the stem carrier bracket 50 are resisted by the drive screw 64 engaged with the stem carrier bracket 50. The cantilevered arm 50 in its engagement with the bearing track 60 can bind, instead of sliding, due to misalignment and slight mechanical obstructions. Any increase in the coefficient of friction which along with the resulting coupling effect of the forces shown in FIGS. 3B and 3C tends to cause a seizing rather than sliding between the stem carrier bracket 50 and the bearing truck 62. The motion of raising and lowering the pedestal, shown by arrows 84 in FIG. 3 results in the pedestal stem 38 and stem carrier bracket 50 moving in a motion between end positions as shown by the solid lines in FIG. 3A and the dashed lines showing the pedestal 38 and stem carrier bracket 50. Consider a uniform motion reference line 87 showing the horizontal attitude of a perfectly vertical lifting motion of the pedestal and pedestal stem 38, a variation (misalignment) from horizontal is shown near one edge of the pedestal by a dimension 86 which in a 10-inch diameter pedestal can be as much as 0.010-0.012 inches. The variations are not necessarily consistent, process conditions may vary and the forces causing the cantilevered stem carrier bracket to bend, bind, or otherwise change its orientation, may vary as the temperature of the support brackets and support structure change over time. Each variation creates another factor which increases the variation in the vertical orientation and reduces the likelihood of a consistent, repeatable lifting orientation and alignment.
The location of the drive screw on the same side of the slide support as the pedestal stem causes the initial force from the drive screw 64 to bend the stem carrier bracket 50. The force from the drive screw increases or decreases the stress bending the stem carrier bracket 50. Only after this initial bending is there a transfer of force to the bearing truck 62 attached to the end of the stem carrier bracket 50 to move the stem carrier bracket 50 along the bearing track 60. This arrangement causes a slight but unpredictable variation in the pedestal position depending on how much the stem carrier bracket is bent and/or how much misalignment there is between the threaded rod and the threaded opening in the stem carrier bracket 50 and the misalignment between the threaded rod and the bearing track 60.
FIGS. 3B and 3C illustrate idealized schematic views of the forces experienced by a stem carrier bracket 50 as shown in FIG. 3A. In FIG. 3B the stem carrier bracket 50 is mounted through a bearing truck 94 to a bearing rail 92 fixed to a support structure. When the process chamber is at ambient pressure the weight of the pedestal represented by the arrow 96 is opposed by the vertical force of the drive screw 64 represented by the arrow 98. The two forces represent a bending of the stem carrier bracket 50 in a direction as shown by the arrow 100. Resistance to that bending is provided by the attachment (not shown) between the bearing truck 94 and the bearing rail 92.
In contrast, the configuration of FIG. 3C shows forces when the process chamber is evacuated thereby causing the pedestal stem to be forced inward (upward) as represented by the arrow 104 and that force being resisted by the drive screw 64 whose downward forces are represented by the arrow 106. The combination of the two forces 104 and 106 will cause a bending in the stem carrier bracket 50 as shown by the arrow 108. Again the connection between the sliding bearing truck 94 and fixed bearing rail 92 resist the forces tending to rotate the stem carrier bracket 50 from its horizontal attitude.
As the sizes of substrates to be processed increase geometric variations in the process chamber dimensions need to be minimized to avoid variations in process conditions and process performance across the width of a substrate. Therefore it is desirable to minimize the variation and deviation from parallel as a pedestal and stem are lifted and provide consistency and repeatable performance based on component orientation and configurations which can be consistently predicted and repeated.